Normal human B lymphocytes require the fulfillment of several distinct biological criteria prior to undergoing a proliferative response. In addition to an obligate antigen specific activation step, necessary for the acquisition of specific growth factor receptors, cells of B lineage require subsequent interaction with a specific growth factor ligand. One ligand responsible for promoting G1 phase cell cycle progression and S phase entry has been termed B cell growth factor. The moiety possessing B cell specific growth factor activity has been shown to be a relatively heat labile trypsin sensitive protein of 12-14 kD with a major isoelectric point of 6.3-6.6. This protein has been shown to possess cellular specificity and functional specificity. Both the biological and biochemical characterization of this important lymphokine would be greatly expedited by the availability of a homogenously purified reagent in relatively abundant supply. We therefore propose to purify the 12-14 kD BCGF moiety to homogeneity and to examine the biological and biochemical characteristics of the purified reagent. We also propose to prepare this growth factor by means of recombinant technology employing the use of both prokaryotic and eukaryotic expression vectors. In addition, both heterotypic and monoclonal antisera will be prepared against the 12-14 kD protein which should provide an important component for both affinity based purification and for multiple biological investigations.